The present invention relates in general to fluorescent lamps and in particular to fluorescent lamps the glass envelopes of which include a transparent electrically conductive material that serves to reduce the open circuit voltage required to start the fluorescent lamps.
The operation of fluorescent lamps is well understood by those skilled in the art, but certain salient features of the operation are reviewed here for the purposes of facilitating a better appreciation of the benefits of the present invention.
A fluorescent lamp typically comprises a sealed glass envelope, usually in the form of a glass tube, that contains a small amount of mercury and an inert gas under low pressure. Examples of inert gases that can be used are argon, krypton, neon, xenon and mixtures thereof. The inside surface of the glass envelope is coated with a phosphor powder. Two electrodes are located within the glass envelope and are wired to an electric circuit that is connected to an alternating current supply.
When the fluorescent lamp is turned on, electric current flows through the electric circuit causing electrons to be emitted from the electrodes. The electrons then flow through the interior of the glass envelope along an electrical field applied between the two electrodes. In the meantime, at least a portion of the liquid mercury is vaporized to mercury gas within the glass envelope, and the electrons ionize the mercury gas. This increases the conductivity of the inert gas so that more electric current can flow and more power thereby dissipated in the inert gas. The power so provided converts additional liquid mercury into a gas until a near-optimal pressure of mercury vapor has been established by the evaporation of the liquid mercury. As electrons and mercury ions move through the interior of the glass envelope, the electrons collide with the gaseous mercury atoms. As a result, the energy level of the outermost electron in some of the gaseous mercury atoms is raised. When these electrons return to their original energy levels, photons are radiated.
Most of the photons radiated by the gaseous mercury atoms are in the ultraviolet wavelength range and are not visible to the naked eye. However, when one of the ultra-violet photons strikes the phosphor powder that coats the inside surface of the glass envelope, one of the phosphor atom's electrons is excited to a higher energy level. When the electron in the phosphor atom returns to its normal energy level, it radiates energy in the form of another photon. The wavelength of this photon is in the visible spectrum and can be seen by the naked eye.
Before the lamp is turned on, there are a very few ions and electrons present in the gas within the glass envelope. Consequently, it is difficult to pass an electric current from one electrode to the other and establish an electric arc between the electrodes capable of sustaining the generation of white light within the glass envelope. Therefore, it is necessary to initially provide a high enough voltage across the electrodes to generate a sufficient density of free electrons in the gas within the glass envelope such that the gas becomes an electrically conductive medium. Once that is accomplished, the current across the electrodes increases to a level capable of establishing the electric arc required for normal lamp operation. The desired voltage and operating current are provided and controlled by a ballast that is incorporated into the electric circuit to which the lamp is wired. The voltage across the electrodes required for the lamp to ignite and transition to arc is variously referred to as the ignition voltage and starting voltage. The voltage that is supplied to the ballast during lamp ignition is usually referred to as the open circuit voltage. It is generally desired for design simplicity and the cost of the ballast that the ignition voltage required to start the lamp be as low as possible.
The operation of ballasts are well known to those skilled in art, and the details of their operation are not presented here. However, it is noted that with a so-called rapid-start type of ballast a heating current continually flows through both electrodes while the ignition voltage is applied between the two electrodes. When the fluorescent lamp is turned on, the voltage that is applied must exceed the ignition voltage that is required to ionize the gas in the glass envelope so that the electric arc current can be established. After the ballast is switched on, the filaments of both electrodes heat up very quickly (due to the heating current), thereby thermionically emitting the electrons required to sustain the operating current of the lamp without unduly damaging the electrodes. A related type of ballast, known as a programmed-start ballast, provides a similar heating current to the electrodes during ignition, and for a few seconds thereafter, until the electrodes have reached the temperature necessary for the thermionic emission of electrons required to establish and supply the electric arc. Thereafter, the heating current is terminated to save the portion of power supplied to the lamp for heating and the electrodes are self-heated by both the lamp current and thermal contact with the discharge. Both rapid-start and programmed-start ballasts are advantageous for long lamp life because, during start-up and warm-up of the lamp, most of the electrons supplied by the electrodes are from thermionic emission as opposed to a more destructive process.
Another type of ballast that is used with fluorescent lamps is the so-called instant-start ballast. This type of ballast applies a somewhat higher voltage across the electrodes than a rapid-start ballast but does not simultaneously heat the electrodes. The electrons required by the electric arc during ignition and briefly thereafter are primarily emitted from the electrodes by a damaging process of secondary electron emission. This emission is driven by bombardment of the electrode with high energy ions. The high energy ions not only eject electrons from the cold electrodes but they also sputter the emission mix and the tungsten metal from the electrodes, typically resulting in reduced lamp life. Consequently, to achieve the longest possible lamp life, it is desirable to enable all lamps to start on rapid-start and programmed-start ballasts which provide a lower circuit voltage than the instant-start ballasts.
There can be instances where fluorescent lamps, for various reasons, are difficult to start or transition to arc. For example, the ballast available in a particular instance may not be capable of generating an open circuit voltage that exceeds the ignition voltage required for the lamp to ignite.
Standard fluorescent lamps utilizing only argon as the inert gas filler have a lower lumen efficacy, expressed as lumens per watt, as compared to mixed argon/krypton energy-efficient, lower wattage fluorescent lamps. These lower wattage lamps yield reduced positive column power through substitution of some or all of the argon fill gas by krypton, or possibly xenon. The addition of krypton reduces energy consumption in fluorescent lamps because krypton has a higher atomic weight than argon resulting in a lower voltage gradient in the positive column with lower heat conduction losses per unit length of discharge in the lamp. Lamps of this type are often referred to as watt-miser lamps. The addition of krypton increases the open circuit voltage required to start the lamp so that the lamp will not start with some ballasts including many rapid-start and programmed-start ballasts. For example, a standard full-argon F32T8 lamp of the General Electric Company requires an open circuit voltage of approximately 300 to 315 volts to ignite while an argon-krypton F32T8WM watt-miser fluorescent lamp of the General Electric Corporation may require an open circuit voltage of more than about 400 volts to transition to arc. In these lamp designations, “F” means fluorescent, “32” means 32 watts dissipated in the lamp, “T8” means a lamp having a diameter of eight one-eighths of an inch, or one inch, and “WM” means “watt-miser”. The energy-saving argon-krypton lamps provided by manufacturers other than the General Electric Company have similar identifying nomenclature.
If the watt-miser fluorescent lamps are used with the instant-start ballasts described above that are capable of providing open circuit voltages in excess of about 400 volts, the watt-miser lamps normally will not experience any difficulty in igniting. However, if the watt-miser fluorescent lamps are paired with the rapid-start or programmed-start ballasts described above, that are only capable of providing open circuit voltages of less than 400 volts, (but which typically provide for longer lamp life) the watt-miser fluorescent lamps may not ignite. It would be desirable to have watt-miser fluorescent lamps available that, in general, start and transition to arc for rapid-start and programmed-start ballasts as well as for instant-start ballasts. And in particular, it would be desirable to have available a high-efficiency lamp containing krypton capable of starting and operating on all existing ballasts so that the lamps can be rated for “Universal Operation On All Ballasts”.
Certain of the foregoing concerns have been addressed in the prior art by the provision of a starting assembly or starting aid that effects starting of fluorescent lamps with or without krypton in the fill gas. The starting assembly provides an easier path for the electrons to flow along during start-up of the lamps, thereby reducing the peak starting voltage requirement of the lamps.
One conventional starting aid consists of a metal strip attached to the outside of the glass envelope of the lamp. In a typical embodiment, an optically-opaque electrically-conducting metal strip is applied to the outside of the glass envelope of a lamp having a diameter of about one and one-half inches. The strip is approximately one-fourth inch wide and extends the full length of the lamp. A disadvantage with this starting aid is that the metal strip covers a relatively large percentage (approximately five percent) of the exterior surface of the glass envelope. The strip, therefore, absorbs or reflects approximately five percent of the light emitted by the lamp. Even though some of the light reflected by the metal strip is redistributed inside the lamp and is re-emitted, the total emission of the lamp is reduced by more than one to two percent. Another disadvantage is that the metal strip is visible at significant distances. A further disadvantage arises because the strip is usually manually attached to the lamp with an adhesive and an insulating cover to protect against electric shock. This manual process significantly increases the costs of manufacture.
Another starting aid disclosed in the prior art consists of a conductive coating, such as tin oxide, that is applied to the entire inside surface of the glass envelope. As with the opaque metal strip referred to above, a disadvantage with this starting aid is that it absorbs an important quantity (more than approximately one to two percent) of light emitted by the lamp. This is because the tin oxide covers essentially all of the interior surface area of the glass envelope. Another disadvantage is that the tin oxide coating creates potential concerns related to safety and lamp breakage during the manufacturing process. A further disadvantage from an environmental standpoint is that a corrosive agent is required to be used during the coating of the glass envelopes.
Yet another starting aid that is used is the metal luminaire into which the lamp is mounted. However, the proximity of the luminaire to the lamp electrodes is important in that case and the greater the separation, the less efficient is the starting aid. The present invention relaxes the requirements associated with the distance between the lamp electrodes and the metal luminaire and even enables the use of non-conducting (e.g. plastic) luminaires or even the elimination of the luminaire.